Vehicles such as motor cars are each equipped with a lead battery as a secondary battery, and it is used as energy source of engine starting, lighting, fuel injection, etc. It is desired to anticipate the fall of output of the lead battery. If the battery does not have a sufficient power supply after the engine is stopped, it will not be able to restart the engine.
A usual 12-volt lead battery consists of six 2-volt cells connected in series. If any of the cells malfunctions and falls into the state of inverse charge, a voltage drop equal or greater by 2 volts or more than the normal state arises at the time of discharging and the battery may not be able to start the engine.
A method of anticipating the life span of a battery based on the measurement of its voltage recovery characteristics has been proposed.
A method for assessing the deterioration of battery performance based on the voltage recovery characteristics on the time axis after a large load, is described in Japanese Patent Laid-Open No. H11 (1999)-52034 (paragraph 0011 and FIG. 2).
The technique of the above-mentioned patent publication bases the assessment of performance deterioration on the time expended until the voltage at large load application is restored, and therefore the battery needs to have virtually equal load currents before and after the large load application. A car battery has no load before the engine is started, and it is charged after the engine has started. Due to a charging current after the large load application, the battery voltage soon exceeds the no-load voltage, and therefore the scheme of this patent publication cannot be used for the car battery.
In regard to the battery for the driving power source of hybrid cars (using an internal-combustion engine and a motor) and fuel cell cars, the battery characteristics cannot be measured during a run. For this reason, it is difficult to estimate a deficiency of the power supply during a run and the failure of engine restarting etc.
The present invention is intended to solve the foregoing problems, and to provide a method and apparatus for the performance assessment (monitoring) of a secondary battery for anticipating the abrupt fall of battery output.